The following relates to the illumination arts, lighting arts, solid-state lighting arts, and related arts.
Incandescent and halogen lamps are conventionally used as both omni-directional and directional light sources. Omnidirectional lamps are intended to provide substantially uniform intensity distribution versus angle in the far field, greater than 1 meter away from the lamp, and find diverse applications such as in desk lamps, table lamps, decorative lamps, chandeliers, ceiling fixtures, and other applications where a uniform distribution of light in all directions is desired.
With reference to FIG. 1, a coordinate system is described which is used herein to describe the spatial distribution of illumination generated by an incandescent lamp or, more generally, by any lamp intended to produce omnidirectional illumination. The coordinate system is of the spherical coordinate system type, and is shown with reference to an incandescent A-19 style lamp L. For the purpose of describing the far field illumination distribution, the lamp L can be considered to be located at a point L0, which may for example coincide with the location of the incandescent filament. Adapting spherical coordinate notation conventionally employed in the geographic arts, a direction of illumination can be described by an elevation or latitude coordinate and an azimuth or longitude coordinate. However, in a deviation from the geographic arts convention, the elevation or latitude coordinate used herein employs a range [0°, 180°] where: θ=0° corresponds to “geographic north” or “N”. This is convenient because it allows illumination along the direction θ=0° to correspond to forward-directed light. The north direction, that is, the direction θ=0°, is also referred to herein as the optical axis. Using this notation, θ=180° corresponds to “geographic south” or “S” or, in the illumination context, to backward-directed light. The elevation or latitude θ=90° corresponds to the “geographic equator” or, in the illumination context, to sideways-directed light.
With continuing reference to FIG. 1, for any given elevation or latitude an azimuth or longitude coordinate can also be defined, which is everywhere orthogonal to the elevation or latitude θ. The azimuth or longitude coordinate θ has a range [0°, 360°], in accordance with geographic notation.
It will be appreciated that at precisely north or south, that is, at θ=0° or at θ=180° (in other words, along the optical axis), the azimuth or longitude coordinate has no meaning, or, perhaps more precisely, can be considered degenerate. Another “special” coordinate is θ=90° which defines the plane transverse to the optical axis which contains the light source (or, more precisely, contains the nominal position of the light source for far field calculations, for example the point L0).
In practice, achieving uniform light intensity across the entire longitudinal span φ=[0°, 360°] is typically not difficult, because it is straightforward to construct a light source with rotational symmetry about the optical axis (that is, about the axis θ=0°). For example, the incandescent lamp L suitably employs an incandescent filament located at coordinate center L0 which can be designed to emit substantially omnidirectional light, thus providing a uniform intensity distribution respective to the azimuth θ for any latitude.
However, achieving ideal omnidirectional intensity respective to the elevational or latitude coordinate is generally not practical. For example, the lamp L is constructed to fit into a standard “Edison base” lamp fixture, and toward this end the incandescent lamp L includes a threaded Edison base EB, which may for example be an E25, E26, or E27 lamp base where the numeral denotes the outer diameter of the screw turns on the base EB, in millimeters. The Edison base EB (or, more generally, any power input system located “behind” the light source) lies on the optical axis “behind” the light source position L0, and hence blocks backward emitted light (that is, blocks illumination along the south latitude, that is, along θ=180°), and so the incandescent lamp L cannot provide ideal omnidirectional light respective to the latitude coordinate.
Commercial incandescent lamps, such as 60 W Soft White incandescent lamps (General Electric, New York, USA) are readily constructed which provide intensity across the latitude span θ=[0°, 135°] which is uniform to within ±20% (area D) of the average intensity (line C) over that latitude range as shown in FIG. 2. Plot A shows the intensity distribution for an incandescent lamp with a filament aligned horizontally to the optical axis, and plot B shows the intensity distribution for an incandescent lamp with a filament aligned with the optical axis. This is generally considered an acceptable intensity distribution uniformity for an omnidirectional lamp, although there is some interest in extending this uniformity span still further, such as to a latitude span of θ=[0°, 150°] with ±10% uniformity. These uniformity spans would be effective in meeting current and pending regulations on LED lamps such as U.S. DoE Energy Star Draft 2, and U.S. DoE Lighting Prize.
By comparison with incandescent and halogen lamps, solid-state lighting technologies such as light emitting diode (LED) devices are highly directional by nature, as they are a flat device emitting from only one side. For example, an LED device, with or without encapsulation, typically emits in a directional Lambertian spatial intensity distribution having intensity that varies with cos(θ) in the range θ=[0°, 90°] and has zero intensity for θ>90°. A semiconductor laser is even more directional by nature, and indeed emits a distribution describable as essentially a beam of forward-directed light limited to a narrow cone around θ=0°.
Another challenge associated with solid-state lighting is that unlike an incandescent filament, an LED chip or other solid-state lighting device typically cannot be operated efficiently using standard 110V or 220V a.c. power. Rather, on-board electronics are typically provided to convert the a.c. input power to d.c. power of lower voltage amenable for driving the LED chips. As an alternative, a series string of LED chips of sufficient number can be directly operated at 110V or 220V, and parallel arrangements of such strings with suitable polarity control (e.g., Zener diodes) can be operated at 110V or 220V a.c. power, albeit at substantially reduced power efficiency. In either case, the electronics constitute additional components of the lamp base as compared with the simple Edison base used in integral incandescent or halogen lamps.
Yet another challenge in solid-state lighting is the need for heat sinking. LED devices are highly temperature-sensitive in both performance and reliability as compared with incandescent or halogen filaments. This is addressed by placing a mass of heat sinking material (that is, a heat sink) contacting or otherwise in good thermal contact with the LED device. The space occupied by the heat sink blocks emitted light and hence further limits the ability to generate an omnidirectional LED-based lamp. This limitation is enhanced when a LED lamp is constrained to the physical size of current regulatory limits (ANSI, NEMA, etc.) that define maximum dimensions for all lamp components, including light sources, electronics, optical elements, and thermal management.
The combination of electronics and heat sinking results in a large base that blocks “backward” illumination, which has heretofore substantially limited the ability to generate omnidirectional illumination using an LED replacement lamp. The heat sink in particular preferably has a large volume and also large surface area in order to dissipate heat away from the lamp by a combination of convection and radiation.
Currently, the majority of commercially available LED lamps intended as incandescent replacements do not provide a uniform intensity distribution that is similar to incandescent lamps. For example, a hemispherical element may be placed over an LED light source. The resultant intensity distribution is mainly upward going, with little light emitted below the equator. Clearly, this does not provide an intensity distribution, which satisfactorily emulates an incandescent lamp.